While demand for power is less at midnight than in the daytime, a power generator can be used more efficiently when it is continuously operated. Since generated power is difficult to store, the power generation capability of power generating facilities is set in accordance with peaks of power demand. Under such circumstances, it is well known that power costs less at midnight than in the daytime. At places of power consumption, such as homes, companies, factories, and the like, it is desirable that midnight power that has been stored in storage type batteries be used in the daytime to thereby level out reduction in power charges and a load.
Japanese Patent Laying-Open No. 2000-32669 (PTL1) discloses a technique wherein a storage battery is introduced to a home to store midnight power with a charger, and this power is drawn from the storage battery in the daytime for use as a household power supply.